The present invention relates to aluminum alloys, and more particularly to a 7000 series alloy of the aluminum-zinc-magnesium-copper type characterized by high strength, high fatigue properties and high fracture toughness.
A significant economic factor in operating aircraft today is the cost of fuel. As a consequence, aircraft designers and manufacturers are constantly striving to improve the overall fuel efficiency. One way to increase this fuel efficiency, as well as overall performance, is to reduce structural weight. Since aluminum alloys are used in a large proportion of the structural components of most aircraft, significant efforts have been expended to develop aluminum alloys that have higher strength to weight ratios than the alloys in current use, while maintaining the same or higher fracture toughness, fatigue resistance and corrosion resistance.
For example, one alloy currently used on the upper wing skins of some commercial jet aircraft is alloy 7075 in the T651 temper. Alloy 7075-T651 has a high strength to weight ratio, while exhibiting good fracture toughness, good fatigue properties, and adequate corrosion resistance. Another currently available alloy sometimes used on commercial jet aircraft, alloy 7178-T651, is stronger than 7075-T651; however, alloy 7178-T651 is inferior to alloy 7075-T651 in fracture toughness and fatigue resistance. Thus there are more restrictions to taking advantage of the higher strength to weight ratio of alloy 7178-T651 without sacrificing fracture toughness and/or fatigue performance of the component on which it is desired to use the alloy. Other currently available alloys and tempers, although sometimes exhibiting good toughness properties and high resistance to stress-corrosion cracking and exfoliation corrosion, offer no strength advantage over alloy 7075-T651. Examples of such alloys are 7475-T651, T7651 and T7351 and 7050- T7651 and T73651. Thus, with currently available alloys and tempers, it is impossible to achieve a weight saving in aircraft structural components while maintaining fracture toughness, fatigue resistance and corrosion resistance at or above the level currently available with alloy 7075-T651.
It is therefore an object of the present invention to provide an aluminum alloy for use in structural components of aircraft that has a higher strength to weight ratio than the currently available alloy 7075-T651. It is a further object of the present invention to provide such an alloy that exhibits improved fatigue and fracture toughness properties while maintaining stress-corrosion resistance and exfoliation corrosion resistance at a level approximately equivalent to that of alloy 7075-T651.